I think age also has a lot to do with it. Most of us were still teenagers or young kids when the 2k3 series came out, (hell, I was only 16 when the 2k3 series was announced!), and the Nick show started 5 years ago in 2012 and again we were all of course 5 years younger than we are now.

Nowadays most of the TMNT fanbase is in their late 20's or early 30's, so we're all older and get less hyped over kids cartoons. I imagine though the younger fans whose first show was the Nick cartoon might be more interested.

I don't think being 5 years younger makes much difference when you are in your late 20's/early 30's

As for the Nick toon, some of the kid fans are still kids, so they get two childhood incarnations.